import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-3, 3, 50)
y = 2 * x + 1

plt.figure(num=1, figsize=(8, 5), )
plt.plot(x, y, )
# plt.show()

ax = plt.gca()
ax.spines['right'].set_color('none')
ax.spines['top'].set_color('none')
ax.spines['top'].set_color('none')
ax.xaxis.set_ticks_position('bottom')
ax.spines['bottom'].set_position(('data', 0))
ax.yaxis.set_ticks_position('left')
ax.spines['left'].set_position(('data', 0))

# plt.show()

# 然后标注出点(x0, y0)的位置信息.
# 用plt.plot([x0, x0,], [0, y0,], 'k--', linewidth=2.5)
# 画出一条垂直于x轴的虚线.
x0 = 1
y0 = 2 * x0 + 1
plt.plot([x0, x0, ], [0, y0, ], 'k--', linewidth=2.5)
# set dot styles
plt.scatter([x0, ], [y0, ], s=50, color='b')
# plt.show()


plt.annotate(r'$2x+1=%s$' % y0, xy=(x0, y0), xycoords='data', xytext=(+30, -30),
             textcoords='offset points', fontsize=16,
             arrowprops=dict(arrowstyle='->', connectionstyle="arc3,rad=.2"))

plt.text(-3.7, 3, r'$This\ is\ the\ some\ text. \mu\ \sigma_i\ \alpha_t$',
         fontdict={'size': 16, 'color': 'r'})

plt.show()
